Progress on PVY

New strains of the PVY virus are migrating across the country and that has both the potato industry and researchers looking for varieties with more pronounced resistance to the virus strains and for less expensive means to test for the different strains of the virus.

On September 1, 2009, a team of researchers across the country initiated a five year grant to development better management of PVY and eradicate PVY necrotic strains. They will be evaluating varieties that show greater resistance to these new strains of PVY.

Stewart Gray, Ph.D., a plant pathologist at Agriculture Research Service in Ithaca, N.Y. and at Cornell University, is the principal investigator on the grant with 8 additional co-P.I.s from Idaho and Wisconsin.

Gray said that the project has three particular goals: 1) trying to help seed certification agencies to develop better practices to detect and eliminate PVY from the seed stocks, 2) working with growers overall in management of the disease on the farm. Reducing transmission, increasing their efficiency of rouging on the farm to further reduce the amount of PVY in the system, and 3) working with breeding programs to develop varieties with greater PVY resistance.

In the mean time other researchers are trying to develop better means of testing and identifying the PVY mutations and one of the keys to unlocking the identity of the mutations is found in bacteria living in the thermal pools of Yellowstone National Park.

PVY O, the O stands for Ordinary or common strain, also known as common mosaic has been a problem for potato growers for more then 100 years. PVY O produces yield loss but has no effect on tuber quality.

In the early 1990s a new strain of PVY showed up in eastern Canada, PVY N. The N stands for Necrotic. This virus causes vein necrosis in tobacco and milder symptoms in potatoes. Tobacco and potatoes, along with tomatoes and capsicum peppers are all members of the nightshade family. These newer mutations are not only affecting yields but tuber quality.

PVY is a single strand RNA virus and therein lies the inherent problem with the virus.

Potato researcher Phil Nolte, Ph.D., explains that because PVY is just a single strand of RNA, it can break apart and reassemble into multiple combinations.

The virus could have spontaneously mutated, said Nolte explaining how we have gone from a single PVY O strain to PVY N, PVY NTN, also referred to as N strain Tuber Necrotic, and PVY N:O, also described as PVY N:Wilga in Europe, currently identified strains during the past 20 years..

Change is taking place in everything out there, said Nolte.

You hear about new species of weeds that have developed resistance to herbicide, you hear about insects that have developed resistance to insecticides. Change is taking place constantly, so change is taking place in PVY, Nolte said explaining how these new strains have come to be.

Nolte said that varieties like Norkotah, Sheppady, Gem Star, Gem, and Silverton do not exhibit overt symptoms of PVY thereby making it difficult for seed growers to detect the virus and remove it from their fields.

Theres a handful of varieties that were developed over the last 10 or 12 years that do not show very good symptoms of PVY O and makes them a terrific challenge for seed producers because they cant see the virus to remove it, Nolte said.

According to Nolte theres been one case of PVY N. Around 1998 or 1999 a grower in a Western state called Nolte saying that 30% of his Ranger Russets were infected with PVY N. Rangers are hyper-sensitive to the PVY O virus and even though potatoes have a milder response to PVY N, the Rangers exhibited symptoms from PVY N.

Since that initial outbreak, among Ranger Russets, theres been no other reports of PVY N and within a couple of years researchers had developed a means to test for PVY N when PVY N strain Tuber Necrotic began showing up.

It has all the characteristics of PVY N, said Nolte, milder symptoms in potato, veinal necrosis in tobacco, when you challenge these plants with it, but it has this added characteristic, where in some varieties it will cause tuber necrosis.

Currently Idaho seed growers send samples of their stock to Brawley, California for winter testing using a test called ELISA to determine the amount of PVY strains in their stock. ELISA, Enzyme Linked Immuno-Sorbent Assay, is a seralogical technique to find the PVY virus.

According to Nolte the problem with ELISA is that the test cannot differentiate all strains of PVY, thereby requiring more expensive tests to identify the type of strain.

We just dont have a rapid diagnosis technique, said Nolte. Its almost going to have to be a PCR (Polymerase Chain Reaction) technique because of the variability in this virus. Now not only are we finding PVY NTN, were also finding PVY N:O and that seems to be the predominant strain, but more and more were picking up these N:O types which seem to be recombinants.

Nolte described the PCR testing technique as one using an enzyme isolated from a thermophilic bacteria, Thermus aquaticus, first discovered by researchers in Yellowstone National Park.

It is quite laborious and expensive and time consuming but the results are that it is very accurate and allows you to do the differentiation, Nolte said.

Meanwhile, researchers at Agricultural Research Service sites in Ithaca, N.Y. and Aberdeen, Idaho, and at the University of Wisconsin in Madison and the University of Idaho in Moscow, Aberdeen and Idaho Falls, started a five-year research grant this past September with the proposed objective to refine the current PVY management strategies used by growers and seed certification agencies to reduce virus levels in seed stocks and spread of virus within the crop.

At the Aberdeen ARS center plant pathologist Jonathan Whitworth, PhD, is testing existing potato varieties, new varieties and advanced breeding lines with the new strains of PVY to see if they produce tuber symptoms, tuber necrosis or if they show resistance to the virus.

Whitworth said that PVY strains are commonly transmitted by aphids and that one way to lower transmission of the virus by aphids is by planting crop borders around a seed field. Planting winter wheat, soybeans or another crop that stays green while the potatoes are green extends the margin of the potato field and serves as a perimeter defense against aphids.

Aphids are the vector of the virus, said Whitworth, the PVY is carried within the tip their stylets and as they hit a plant and probe it theyll lose some of those virus particles. The more plants they hit, theyll lose that ability to infect and the theory is that it does work on small lots, by the time they get to the potatoes, theyre basically clean again.

Whitworth said that his role in the research grant is to establish a regional PVY screening trial. He will be receiving promising PVY resistant advanced breeding lines from breeding programs throughout the U.S. Then they will trial test them at Idaho, Wisconsin and New York.

Researchers will document the symptoms and susceptibility of the new advanced breeding lines. He called it a coordinated effort that will help us identify and characterize the resistance of new varieties that are coming out.

Well have four years of trials on a regional basis, said Whitworth. Were starting them in 2011. What were doing this summer is were lining up the seed. Were lining up the interest from the breeders and getting their advanced breeding lines ready to go to three sites next year.

Thats a really big thing, said Whitworth, because its a coordinated effort and information will go out to the industry so that they know what the symptoms are like in these newer varieties. They know what the susceptibility is like and the symptoms can be different depending on where theyre grown.

A variety can still be susceptible to PVY, but if it doesnt produce tuber symptoms, its only a yield problem because PVY reduces yield. If the PVY strain produces tuber symptoms, then its a yield and quality problem. Then tubers become unmarketable, Whitworth said.

Whitworth said that although Russet Burbank and Russet Norkotah are susceptible to PVY, the have proven to be highly resistant to tuber necrosis from strains of PVY, as have other varieties. However the Yukon Gold, Yukon Gem, Highland Russet, Alturas and to a lesser extent the Ranger Russet are more susceptible to the tuber necrosis caused by PVY strains.